1. Technical Field
The present invention relates to an image display device and a projector.
2. Related Art
The image quality has been recently improved remarkably in electronic display devices, e.g., LCDs (Liquid Crystal Displays), EL (Electro-luminescence) displays, plasma displays, CRTs (Cathode Ray Tubes), and projectors. Some devices are about to achieve the performance capabilities about as good as human visual characteristics specifically in terms of resolution and color range. As to a brightness dynamic range, however, the reproduction range is of about 1 to 102 [nit], and the number of tones is generally 8 bits. On the other hand, the human vision covers the range of about 10-2 to 104 [nit] for the brightness dynamic range perceivable at a time, and in terms of brightness discrimination capabilities, covers the range of 0.2 [nit], which is corresponding to about 12 bits if converted into the number of tones. If users having such visual characteristics look at display images on currently-available display devices, they may feel something is missing, i.e., display images are not close to reality enough, and lacking visual impact. This is caused because the users easily notice the narrow brightness dynamic range and the lacking of tones in portions of shadow and highlight.
With CG (Computer Graphics) for use in movies, games, and others, pursuing reality for graphical representation has been becoming mainstream by providing display data (hereinafter, referred to as HDR (High Dynamic Range) display data) with brightness dynamic range and tone characteristics both close to those of human vision. The problem for such pursuit is that the currently-available display devices are not capable enough to display the resulting CG contents, and thus the CG contents cannot be displayed expressive enough as they are supposed to.
Another issue here is that the next OS (Operating System) is planning to take the 16-bit color space, and compared with the current 8-bit color space, the dynamic range and the number of tones are expected to be dramatically increased. This thus may lead to more demands for electronic display devices with wider dynamic range and larger number of tones to make full use of the 16-bit color space.
Some display devices, i.e., projection-type display devices (projectors) such as liquid crystal projectors, DLP™ (Digital Light Processing) projectors are capable of large-screen display, and serve effective to reproduce the display images with reality and impact. In the field of display devices, the following proposals are made to solve the above-described problems.
Patent Documents 1 to 3 (JP-A-2001-100689, JP-T-2004-523001, and JP-A-6-167690) describe the basic configuration for widening the brightness dynamic range in projectors. That is, a luminous flux ejected from a light source is separated into color lights of red, green, and blue, and the resulting color lights are each modulated by a first optical modulator so that any desired illumination distribution is formed. After modulation as such, the resulting color lights are combined by a dichroic prism and then transmitted to a second optical modulator by a relay optical system for brightness adjustment thereby before illumination. The first and second optical modulators are often of the same dimension, and there thus needs to set the relay optical system by the magnification of 1.
In such a configuration, a light-transmissive modulator is used as the optical modulator, a typical example of which is a liquid crystal light valve. The light-transmissive modulator is of the pixel structure or the segment structure in which the transmittance can be independently controlled, and control can be exercised over a two-dimensional transmittance distribution. As an alternative to such a light-transmissive modulator, a reflection modulator is a possible option, a typical example of which is a DMD (Digital Micromirror Device).
Considered here is a case of using an optical modulator having the transmittance of 0.2% for dark display, and the transmittance of 60% for bright display. If with a piece of optical modulator, the brightness dynamic range is 60/0.2=300. With the above-described display device, the brightness dynamic range is equivalent to 300 pieces of optical modulators arranged optically in series. It thus means, theoretically, that the brightness dynamic range of 300×300=90000 is realized. This is similarly applicable to the number of tones, and by arranging 8-bit-tone optical modulators optically in series, the resulting number of tones can exceed 8 bits.
Note here that the first and second optical modulators are separately driven by a modulation signal derived from a video signal.
The problem with the projectors described in Patent Documents 1 to 3 is that, however, on the optical path with the relay optical system disposed in the middle, only one side thereof carries thereon a dichroic prism. Due to the dichroic prism disposed as such, the distance from the relay optical system to the first optical modulator is not the same as the distance from the relay optical system to the second modulator system. As such, the optical path conditions are not the same between the sides of the relay optical system, thereby causing a difficulty in designing the relay optical system and resulting in the considerable increase of cost.
The design difficulty easily results in problems of chromatic aberration highly visible in the relay optical system, the lower image-forming performance capabilities, and others. These problems get in the way of increasing the contrast of displaying image, increasing the dynamic range, and the like.